The present invention relates to a separator that heats each board in a book through direct contact between the separator and the board for the manufacture of metal clad laminates, unclad laminates, and multilayer printed circuit boards (hereinafter referred to as PCB).
The electronic packaging industry is experiencing rapid changes in manufacturing technology, moving toward ever higher density and performance circuits. 100 .mu.m and narrower circuit traces are common nowadays, and demand for 50 .mu.m circuit traces is increasing. Similarly, the hole population is ever increasing, and in some applications, such as land grid array in a first level chip packaging, it amounts to several tens per cm.sup.2 board. In order to accommodate such a demand, the size of a land is shrinking, which in turn necessitates a higher degree of dimensional stability in the metal clad laminate or pressed multilayer PCB. Otherwise, the failure rate of the registration of a drilled hole on the land will also increase. Therefore, the variation in dimensional movement of a metal clad laminate or pressed multilayer PCB needs to be minimized to improve the production yield and quality of the processed board. Also, a printed circuit board, such as memory module or high level of impedance control board, requires a very narrow level of thickness tolerance. Therefore, the industry needs a technology that can produce highly uniform laminates or pressed multilayer PCB.
In order to have thermally curable prepregs cured to laminates with high uniformity, the conditions of the curing should result in each prepreg having a similar chemorheological history regardless of its position in the press daylight. The chemorheology of a thermosetting prepreg is affected by three factors: temperature, time, and reaction kinetics. The prepregs in a book can be assumed to be the same. Therefore, the only factor that causes differences between prepregs in a book is temperature during heating cycle. Thus, to make the chemorheologies of the prepregs more identical to each other, the heating of each prepreg should be as similar as possible.
Conventional technology accommodates about 10 boards per book. Each board contains prepregs, metal foils, and innerlayer PCB if multilayer PCB is to be made; and each board is sandwiched between metal separators. Thereafter, the book is loaded into the press daylight, and heated through the platens located at the top and the bottom of the book. Because of this heating mechanism, the thermal history of each board varies according to the location of the board in the book. It is quite common for the temperature difference among the boards to be as high as 15.degree. C. during the heating cycle and for the difference from position to position on a board to range from 3 to 10.degree. C. Therefore, the prepregs undergo different paths of chemorheology, depending upon their location in the book. This results in large variations in the physical qualities of the pressed boards, the so called laminates, a fact well known to those conversant with the PCB industry.
To circumvent the intrinsic problem of the conventional technology, PCT/IT 92/00101, U.S. Pat. No. 5615470 and U.S. Pat. No. 5647940 were disclosed in the early 1990's, and are incorporated herein for reference. These patents utilize metal foil, which is to be clad to the insulating material. The metal foil is continuously wrapped around the insulating material and the boards are separated by insulating separators. The metal foil is connected to electricity to heat the boards through induction. By this means, each of the boards in a book receives the same amount of heat and the temperature difference among the boards is less than 5.degree. C. in most cases. However, the temperature difference on individual boards from position to position is quite often as high as 10.degree. C. In the PCB industry, electrodeposited copper foil is most widely used, and the thickness tolerance is .+-.10% in most cases. This is one of the causes of the high level of temperature differences on a board. Experts within the PCB industry know that no real improvement is made by this technology in achieving a higher level dimensional stability.
Furthermore, because prepregs are built up with metal foil through continuous wrapping, the possibility of creating a surface imperfection is higher in comparison to the conventional technology in which the build up procedure can be divided into several areas, minimizing the dusty air flow of the prepregs onto the surfaces of the metal foil and separator.
In addition, the PCB shop is required to have a matching set of separators for each that match each panel size to minimize the loss in the metal foil; this means a higher investment in separators. The disadvantages of the metal foil technology can be summarized as follows:
1. It is possible to produce only metal clad laminates. No other laminates can be produced. PA1 2. Most prepregs for industrial use are made of glass fabric, and glass fabric prepregs are better handled in a separate room to minimize the surface contamination of the metal foil and the separator. However, there are no other means than to handle the prepregs with the metal foil and separator in the same room and on the same table. PA1 3. It is mandatory to have as many sets of separators as the number of working sizes of the panel. Otherwise, metal foil loss is higher than that in conventional technology. PA1 4. Alignment of intermediate multilayer PCB are done through riveting, but alignment accuracy decreases as the number of layers increases. PA1 5. As the heating source is the metal foil of which the thickness tolerance is +10%, resulting in a temperature variation on an individual board that can be greater than that with conventional technology. PA1 6. A cooling press is required so that the book can be transferred to it after lamination. Otherwise, the cooling takes too long, resulting in poor productivity. But a cooling press induces thermal stress because of the temperature difference on a board from the center to the side. The thermal stress will worsen the dimensional stability of the laminate.
It is known that the physical properties of a laminate can be made consistent if each board in a stack of a book is heated equally. However, the thermal history of each board in a conventional press system differs from board to board in a book. In a recent development, metal foil is used to heat the board directly, achieving a large improvement in the temperature uniformity among boards. However, the temperature difference on an individual board from position to position is worse in most cases than that of the conventional hot press technology where heating is achieved through platens at the top and the bottom.